They argued that Sigg, who was 17 at the time of the killing, was too young to fully understand his actions and decisions.

Sigg also admitted Tuesday to attacking a 22-year-old female jogger at Colorado's Ketner Lake last May. That woman managed to get away.

In all, he pleaded guilty to 15 charges, including first-degree murder, kidnapping, sexual assault of a child, sexual exploitation of a child, and attempted kidnapping, according to The Post.

Sigg, who will be sentenced Nov. 8, faces a minimum of 40 years in prison. He is ineligible for the death penalty because he was a minor at the time of the crime.

Sigg's father Robb issued a statement after Tuesday's hearing.

"We are very thankful that the Ridgeway family will be spared the ordeal of a trial," he said. "We ask the community to continue to support the Ridgeway family and keep them in your thoughts and prayers."

Ridgeway disappeared while walking to school in Westminster, Colo., Oct. 5, 2012. Police say Sigg grabbed the girl and pulled her into his car. Her backpack and some parts of her body were found a few days later at a nearby park. The rest of Ridgeway's remains were later discovered in the crawl space of Sigg's house.